1. Field of the Invention
The present invention relates to an antenna directing apparatus suitable for use with marine satellite communication systems or the like to direct an antenna to a satellite and to an antenna directing apparatus having a rewind function.
2. Description of the Prior Art
FIG. 1 shows an example of a conventional antenna directing apparatus. This antenna directing apparatus is what might be called an azimuth-elevation system. The antenna directing apparatus generally comprises a base 3, an azimuth gimbal 40 mounted on the base 3, an attachment 41 mounted on a U-letter-shaped member 40-2 secured to an upper end portion of the azimuth gimbal 40 and a metal antenna 14 attached to an attachment 41.
The base 3 includes a bridge portion 3-1 that has a cylindrical portion 11 projected upwardly therefrom. A pair of bearings 21-1, 21-2 are provided within the cylindrical portion 11. An azimuth shaft 20 is fitted into the inner rings of the bearings 21-1 and 21-2 and the azimuth gimbal 40 is coupled to the upper end portion of the azimuth shaft 20 through an arm 13.
Thus, under the condition that the azimuth shaft 20 is supported by the bearings 21-1 and 21-2, the azimuth gimbal 40 can be rotated about an axis that passes through the azimuth shaft 20. The azimuth gimbal 40 comprises a lower supporting shaft portion 40-1 and an upper U-shaped portion 40-2. The central axis of the support shaft portion 40-1, i.e., the azimuth axis Z-z is displaced from the axis that passes through the azimuth shaft 20 as shown in FIG. 1. The support shaft portion 40-1 need not be displaced and may be matched with the axis that passes through the azimuth shaft 20.
The U-shaped portion 40-2 of the azimuth gimbal 40 supports therein an attachment 41 of smaller U-letter configuration. The attachment 41 includes elevation shafts 30-1, 30-2 attached to two leg portions 41-1, 41-2, respectively. Proper bearings are respectively mounted on two leg portions of the U-shaped portion 40-2 of the azimuth gimbal 40 and the elevation shafts 30-1 and 30-2 are supported by these bearings so as to be rotatable.
The central axes of the elevation shafts 30-1, 30-2 constitute an elevation axis Y--Y. In this way, the attachment 41 is supported between the two leg portions of the U-shaped portion 40-2 of the azimuth gimbal 40 so as to become rotatable about the elevation axis Y--Y. The elevation axis Y--Y is disposed at a right angle to the azimuth axis Z--Z, and accordingly, is disposed substantially horizontally.
The antenna 14 is mounted on the leg portions 41-1, 41-2 of the attachment 41 of the U-shaped configuration, whereby the antenna 14 can be rotated about the elevation angle line Y--Y together with the attachment 41. The antenna 14 includes the central axis X--X and the central axis X--X is perpendicular to the elevation axis Y--Y.
The attachment 41 has an elevation gyro 44, an azimuth gyro 45, a first accelerometer 46 and a second accelerometer 47. The elevation gyro 44 detects a rotational angular velocity of the antenna 14 rotating around the elevation axis Y--Y. The azimuth gyro 45 detects a rotational angular velocity of the antenna 14 around an axis which is perpendicular both to the elevation axis Y--Y and the central axis X--X of the antenna 14. The first accelerometer 46 detects an inclination angle of the central axis X--X of the antenna 14 about the elevation axis Y--Y. The second accelerometer 47 detects an inclination angle of the elevation axis Y--Y relative to the horizontal plane.
The elevation gyro 44 and the azimuth gyro 45 are not limited, for example, to an integrating type gyro such as a mechanical gyro, an optical gyro or the like and may be an angular velocity detection type gyro such as a vibratory gyro, a rate gyro, an optical fiber gyro or the like.
On one leg of the attachment 41, there is mounted an elevation gear 32 so as to be coaxial with the elevation axis Y--Y. The elevation gear 32 has a pinion 35 meshed therewith and the pinion 35 is attached to a rotary shaft of an elevation servo motor 33 mounted on one leg portion of the U-shaped portion 40-2 of the azimuth gimbal 40.
On the other leg portion of the U-shaped portion 40-2 of the azimuth gimbal 40, there is mounted an elevation angle transmitter 34. The elevation angle transmitter 34 detects a rotational angle .theta. of the antenna 14 around the elevation axis Y--Y and outputs a signal representative of the detected rotational angle.
The azimuth shaft 20 has on its lower end portion an azimuth gear 22. An azimuth servo motor 23 and an azimuth transmitter 24 are attached on the bridge portion 3-1 of the base 3 and pinions (not shown) that are attached to the rotary shafts of the azimuth servo motor 23 and the azimuth transmitter 24 are meshed with the azimuth gear 22.
As shown in FIG. 1, there are provided an elevation angle control loop and an azimuth angle control loop in order to control the antenna directing apparatus. An elevation angle .theta..sub.A assumes an angle formed by the central axis X--X of the antenna 14 and a meridian N on the horizontal plane.
The elevation control loop controls the antenna 14 to rotate about the elevation axis Y--Y so that the elevation angle .theta..sub.A coincides with the satellite altitude angle .theta..sub.S. The elevation angle control loop includes first and second loops. In the first loop, the output of the elevation angle gyro 44 is fed through an integrator 54 and an amplifier 55 back to the elevation angle servo motor 33 so that, even when the ship body rolls and pitches, the angular velocity of the antenna 14 about the elevation axis Y--Y relative to an inertial space is constantly kept zero.
In the second loop, the output signal from the first accelerometer 46 is supplied through an arc sine calculator 57, subtracted by a signal representative of the satellite altitude .theta..sub.S manually set in an adder 57A and then input through an attenuator 56 to the integrator 56 and the amplifier 55. The second loop has a proper time constant so that the elevation .theta..sub.A of the antenna 14 coincides with the satellite altitude angle .theta..sub.S. The attenuator 56 may have an integrating characteristic for compensating for a drift fluctuation of the elevation angle gyro 44.
The azimuth angle control loop has a function to control the azimuth of the azimuth gimbal 40 so that the azimuth angle .phi..sub.A of the antenna 14 coincides with the satellite azimuth angle .phi..sub.S. An output of the azimuth gyro 45 is fed through an integrator 58 and an amplifier 59 back to the azimuth servo motor 23, whereby the antenna 14 can be stabilized when the ship body is turned around the axis Z--Z perpendicular to the central axis X--X of the antenna 14 and the elevation axis Y--Y.
A rotational angle signal providing a rotational angle .phi. of the azimuth gimbal 40 is output from the azimuth transmitter and the rotational angle signal is supplied to an adder 61. In the adder 61, the rotational angle .phi. and a ship's heading azimuth angle .phi..sub.C supplied thereto from a magnetic compass, for example, or gyro compass are added and the satellite azimuth angle .phi..sub.S is subtracted from the sum (i.e., antenna azimuth angle .phi..sub.A). An output signal from the adder 61 is input through an attenuator 60 to the integrator 58. When the sum of the rotational angle .phi. around the azimuth axis Z--Z of the antenna 14 and the ship's heading azimuth angle .phi..sub.C becomes equal to the satellite azimuth angle .phi..sub.S, the azimuth (rotation about the axis Z--Z) of the antenna 14 is settled.
This loop has a proper time constant so that the azimuth angle .phi..sub.A of the antenna 14 coincides with the satellite azimuth angle .phi..sub.S. The attenuator 60 may have an integrating characteristic compensating for the drift fluctuation of the azimuth gyro 45, i.e., the output of the attenuators 56, 60 are equivalent to the output of an integrating type gyro torquer.
In this way, the elevation control loop and the azimuth angle control loop, the central axis X--X of the antenna 14 is directed to the satellite.
In the conventional antenna directing apparatus constructed as above, the signal that is indicative of inclination angle of the central axis X--X of the antenna 14 relative to the horizontal plane from the first accelerometer 46 is supplied to the arc sine calculator 57 and the arc sine is calculated by the arc sine calculator 57 to thereby obtain the elevation angle .theta..sub.A of the antenna 14.
When the satellite altitude angle .theta..sub.S is small, the arc since is calculated at the straight line portion of sine wave so that the elevation angle .theta..sub.A of the antenna 14 can be obtained with relatively high accuracy. However, when the satellite altitude angle .theta..sub.S is large, the arc sine is calculated at the top portion of sine wave so that the calculated result of the elevation angle .theta..sub.A of the antenna 14 is obtained with low accuracy.
Further, since the arc sine of the signal obtained from the first accelerometer 46 is calculated to obtain the elevation angle .theta..sub.A of the antenna 14, it cannot be determined whether or not the elevation angle .theta..sub.A of the antenna 14 exceeds 90.degree.. Therefore, when the elevation angle .theta..sub.A of the antenna 14 exceeds 90.degree., the elevation angle .theta..sub.A of the antenna 14 cannot be controlled accurately.
Consider a transfer function of the azimuth control loop. K assumes a gain of the amplifier 59 and K.sub.T assumes a gain of the attenuator 60. For simplicity, a gain of a driver unit including the azimuth servo motor and a scale factor of the gyro are set to 1 and pitching and inclination of ship body are neglected. The transfer function of the azimuth angle .phi. provided after Laplace transform is expressed by the following equation (1): ##EQU1## where .phi. represents the azimuth angle of the antenna 14, .phi..sub.s represents the satellite azimuth angle, .phi..sub.c represents the gyro compass azimuth angle (ship's heading azimuth angle) and .sub.s represents the Laplace variable. If .phi..sub.C =.phi.'.sub.C /S, .phi..sub.S =.phi.'/S and a final value is calculated, then .phi.-.phi..sub.X '-.phi.'.sub.C. Thus, the azimuth angle .phi.=.phi.+.phi..sub.C of the antenna is directed at the satellite azimuth angle .phi..sub.S.
In the conventional antenna directing apparatus, however, the directed altitude angle of the satellite is changed with latitude or rolling and pitching of ship's body and therefore the elevation angle .theta. of the antenna is also changed. Since the equation (1) includes a term in which a denominator has coefficient Kcos.theta., the frequency characteristic of the azimuth control loop system is changed with the elevation angle .theta. of the antenna. In particular, when the elevation angle .theta. of the antenna is large, the frequency characteristic is deteriorated and a control accuracy of the system is lowered. There is then the drawback that a directing error of the antenna relative to the satellite is increased.
When the elevation angle .theta. of the antenna becomes substantially 90.degree. and the central axis X--X of antenna coincides with the azimuth axis, the azimuth gyro 45 cannot detect the rotational angular velocity of the antenna around the azimuth axis. Consequently, the azimuth control loop cannot function as the servo system and the antenna cannot direct the satellite. This phenomenon is what might be called a gimbal lock.
As shown in FIG. 2, there are provided four servo loops in order to control the antenna directing apparatus. An elevation angle .theta..sub.A of antenna assumes an angle formed by the central axis X--X of antenna 14 relative to the horizontal plane and an azimuth angle .phi. of antenna assumes an angle formed by the central axis X--X of the antenna 14 and the meridian on the horizontal plane.
In the first loop, the output of the elevation gyro 44 is fed through the integrator 54 and the amplifier 55 back to the elevation angle servo motor 33. Thus, even when the ship body is rolled and pitched, the angular velocity of the antenna 14 around the elevation axis X--X can constantly be held at zero.
In the second loop, the output signal from the first accelerometer 46 is supplied through the arc sine calculator 57, subtracted by the signal that instructs the satellite altitude angle .theta..sub.S manually set, for example, and then input through the attenuator 56 to the integrator 54 and the amplifier 55. The second loop has a proper time constant so that the elevation angle .theta..sub.A of the antenna 14 coincides with the satellite altitude angle .theta..sub.S. The attenuator 56 has an integrating characteristic for compensating for a drift fluctuation of the elevation gyro 44. The elevation control loop is formed of the first and second loops.
In a third loop, on the basis of the elevation angle signal .theta. supplied thereto from the elevation angle transmitter 34, 1/cos.theta. calculator 76 calculates 1/cos.theta.. A value which results from multiplying the calculated result with a signal .phi.cos.theta. of the azimuth gyro 45 is fed through the integrator 58 and the amplifier 59 to the azimuth servo motor 23 so that when the ship is turned around the axis Z--Z perpendicular to both the central axis X--X and the elevation axis Y--Y of the antenna 14, the antenna 14 can be stabilized. Also, the frequency characteristic of the azimuth control loop can be made constant regardless of the elevation angle--of the antenna 14.
In a fourth loop, the signal that instructs the rotation angle .phi. of the azimuth gimbal 40 is output from the azimuth transmitter 24. The output signal .phi. is calculated with a satellite azimuth angle .phi..sub.S and the ship's heading azimuth angle .phi. supplied from the magnetic compass or gyro compass, for example, to thereby generate an azimuth error or displacement signal. This azimuth error signal is input through the attenuator 60 to the integrator 58. As a result, at a point where the azimuth angle .phi..sub.A (sum of the rotational angle .phi. of the azimuth gimbal 40 and the ship's heading azimuth angle .phi..sub.C) of the antenna 14 becomes equal to the satellite azimuth angle .phi..sub.S, the azimuth of the antenna 14 is settled.
This loop includes a time constant so that the azimuth angle .phi..sub.A of the antenna 14 coincides with the satellite azimuth angle .phi..sub.S. The attenuator 60 has an integrating characteristic for compensating for the drift fluctuation of the azimuth gyro 45, i.e., the outputs of the attenuators 56, 60 are equivalent to the output of the integrating type torquer. The third and fourth loops constitute an azimuth control loop.
As described above, according to the antenna directing apparatus, under the control of the two control loops formed of four servo loops, the central axis X--X of the antenna 14 can be directed to the satellite direction.
Consider the transfer function of the azimuth control loop. K assumes a gain of the amplifier 59, K.sub.T assumes a proportional gain of the attenuator 60 and K.sub.T /TiS assumes an integrating gain. For simplicity, a gain of the driver unit including the azimuth servo motor 23 and the azimuth gear 22 and the scale factor of the gyro are set to 1 and the pitching of ship body is neglected. The transfer function of the rotational angle .phi. of the antenna after Laplace transform is expressed by the following equations (2) and (3): ##EQU2## where .phi. represents the rotation angle of the antenna 14 around the azimuth axis, .phi..sub.S represents the satellite azimuth angle, .phi..sub.C represents the ship's heading azimuth angle, .theta. represents the rotation angle of antenna 14 about the elevation axis, U.sub.Z represents a fixed error of azimuth gyro, V.sub.I represents the output signal of the integrator 60-2 and S represents the Laplace operator. For example, if .phi..sub.C =.phi..sub.C '/S, .phi..sub.S =.phi..sub.S '/S, U.sub.Z =U.sub.Z =U.sub.Z /S and a final value is calculated, from the equation (3), by substituting the following equation into the equation (1). ##EQU3## we have: ##EQU4## Thus, the fixed error U.sub.Z of the azimuth gyro is compensated for by the integrator 60-2 and the azimuth angle .phi..sub.A (=.phi.+.phi..sub.C) of the antenna becomes equal to the given satellite azimuth angle .phi..sub.S.
In the above conventional antenna directing apparatus, however, since the altitude angle of the satellite to which the antenna is directed is changed with latitude or inclination and also changed largely with rolling or pitching of ship body, the antenna elevation angle .theta. also is changed. In the equation (2), the coefficient 1/cos.theta. is multiplied to the fixed error U.sub.Z of the azimuth gyro so that when the antenna elevation angle .theta. is changed to .theta.', the integrator 60-2 cannot readily follow such change. As a consequence, the rotation angle .phi. generates a transient angle error expressed by substantially U.sub.Z /K.sub.T (1/cos.theta.'-1/cos.theta.). There is then the drawback that the directing error relative to the satellite is increased.
FIG. 3 shows another example of the conventional antenna directing apparatus. In FIG. 3, like parts corresponding to those of FIG. 1 are marked with the same references and therefore need not be described in detail.
In the example of FIG. 3, the elevation angle transmitter 34 is mounted on one leg portion of the U-shaped portion 40-2 of the azimuth gimbal 40. The elevation angle transmitter 34 detects the rotation angle .theta. of the antenna 14 around the elevation axis Y--Y and outputs a signal that corresponds to the detected rotation angle .theta..
In this example, a cable is connected to the antenna directing apparatus. This cable includes a coaxial cable 70 connected to the antenna 14, and lead wires connected to parts mounted on the attachment 41 and the U-shaped portion 40-2. A transmission signal is transmitted to the antenna 14 by means of the coaxial cable 70 and a reception signal is obtained from the antenna 14 through the coaxial cable 70. As shown by a dashed line in FIG. 3, the coaxial cable 70 is extended from the antenna 14 through the attachment 41 the U-shaped portion 40-2 of the azimuth gimbal 40, the support shaft portion 40-1, the arm 13 and along the azimuth shaft 20 to the base 3, from which it is led to the outside.
The cable 70 is made of a flexible material and has a length a little longer than the route extending from the antenna 14 to the base 3. Therefore, when the antenna 14 is rotated about the elevation axis Y--Y and further rotated about the azimuth axis Z--Z, the rotation of the antenna 14 can be prevented from being hindered by the twisting and winding of the cable 70.
However, when the ship body turns or yaws and hence the antenna 14 is rotated about the azimuth axis Z--Z by a large rotational angle, it is frequently observed that the twisting and wrapping of the cable 70 hinder the rotation of the antenna 14. In such case, the antenna directing apparatus includes a rewind mechanism in order to avoid the twisting and wrapping of the cable 70.
As shown in FIG. 3, the rewind mechanism includes a loop formed of the azimuth transmitter 24, a rewind controller 71, a switching circuit 73 and the azimuth servo motor 23. The rewind controller 71 is supplied with the signal that indicates the rotation angle .phi. of the azimuth gimbal 40 output from the azimuth transmitter 24 and supplies a control signal to the switching circuit 73 so that when the antenna 14 is rotated more than 270.degree. from a predetermined reference azimuth, the antenna 14 is rotated 360.degree. in the opposite direction. As described above, the servo motor 23 rotates the azimuth gimbal 40 360.degree. in the opposite direction to thereby untie the twisting of the cable 70.
According to the conventional antenna directing apparatus, when the satellite altitude angle .theta..sub.S is relatively small, even if the ship's body is rolled and pitched, the directing accuracy of the antenna is satisfactory. However, if the ship's body rolls or pitches when the satellite altitude .theta..sub.S is large, the central axis X--X of the antenna 14 and the azimuth axis Z--Z become parallel which causes the so-called gimbal lock phenomenon. If the gimbal lock phenomenon occurs, then the directing accuracy of the antenna is lowered.
Further, in the conventional antenna directing apparatus, if the ship body is in the inclined state such as when the satellite altitude angle .theta..sub.S is large and the ship body is pitched and rolled, when a side wind acts on the ship body, when the cargo is displaced or when a fishing boat draws up a net, then the antenna azimuth angle .phi..sub.A output from the azimuth transmitter 24 contains an error corresponding to the inclination angle of the ship body and finally a large error occurs in the directing azimuth of the antenna 14. Such error becomes remarkable when the inclination of ship body is continued.
FIG. 4 shows an error generating mechanism. The surface 102 (deck) of the ship body rotates at a rotation angle .xi. around the elevation axis Y--Y relative to a horizontal plane 100 (circle having a radius of 1) to form a .xi. inclined surface 101 and also rotates by a rotation angle .eta. around the stern axis OS' of ship body to form a .xi.+.eta. inclined plane 102. An arrow A in FIG. 4 represents a direction vector that directs a satellite 105. This line OS" (length 1) is matched with the central axis X--X of the antenna 14.
Since an angle that is formed by the direction vector A and the horizontal plane 100 is the satellite altitude angle .theta..sub.S (command angle), an angle formed by the direction vector A and the .xi. inclined plane 101 is express as .xi..sub.0 =.angle."OS'=.theta..sub.S -.xi.. The output of the elevation angle transmitter 34 represents the satellite elevation angle .theta. relative to the .xi.+.eta. inclined plane 102. This angle is an angle that is formed by the direction vector A and the ship body plane, i.e., the .xi.+.eta. inclined plane 102. If a perpendicular is extended from the point S" to the .xi. =.eta. inclined plane 102 and the foot of perpendicular is taken as H, the output of the elevation angle transmitter 34 is expressed as .theta.=.angle.S"OH-S"H.
The angle that the direction vector A forms on the horizontal plane 100 with respect to the meridian N is the satellite azimuth angle .phi..sub.S. A point B on the surface of ship's body which also corresponds to the elevation angle axis OB under the condition that the ship body is in the horizontal state is moved to a point B' which satisfies the condition of .angle.S'OB'=90.degree. after inclined .xi.+.eta..
However, since the elevation axis Y--Y passes the point B not the point B' on the surface (deck) 102 of ship body, the angle .angle.S"OB" formed by the elevation axis OB" and the central axis X--X of the antenna 14 is 90.degree..
Accordingly, in the antenna azimuth angle .phi..sub.A detected by the azimuth transmitter 24, there occurs an error B'B"=.DELTA..phi..sub.AE when the ship body surface (deck) 102 is inclined relative to the horizontal plane 100.
If the ship body surface (deck) 102 is inclined, the inclination angle .eta. relative to the horizontal plane 100, the satellite elevation angle relative to the .xi. inclined plane 101 is expressed as .xi..sub.0 =.theta..sub.S -.xi.. This angle is an angle .xi..sub.0 =.angle.S"OS' that is formed by the direction vector A and the ship body surface, i.e., .xi. inclined plane 102. Accordingly, a transmission error .DELTA..phi..sub.AE of the antenna azimuth angle .phi..sub.A detected by the azimuth transmitter 24 is expressed by the following equation (4): EQU .DELTA..phi..sub.AE =tan.sup.-1 {tan .xi..sub.0 .multidot.sin .eta.}(4)
However, the ship body surface (deck) 102 is inclined not only at the inclination angle .eta. but also at .eta.+.xi. relative to the horizontal plane 100. Therefore, as described above, the output of the elevation angle transmitter 34 is the satellite elevation angle .theta. relative to the .xi.+.eta. inclined plane 102. This elevation angle .theta. is the angle formed by the direction vector A and the ship body surface, i.e., the .xi.+.eta. inclined plane 102. At that time, the output of the second accelerometer 47 is not .eta.=BB' but x=B.sub.1 B". Accordingly, the error .DELTA..phi..sub.AE of the antenna azimuth angle .phi..sub.A detected by the azimuth transmitter 24 is calculated by the following equation (5) by using detection amounts .theta. and x instead of .xi..sub.0, .eta. in the equation (2): EQU .DELTA..phi..sub.AE =sin.sup.-1 {sin .theta..multidot.sins.multidot.(cos.sup.2 .theta.s-sin.sup.2 x.multidot.cos.sup.2 .theta.).sup.-1/2 } (5)
where .theta. represents the rotation angle of the antenna around the elevation axis relative to the azimuth gimbal, x represents the inclination angle of the elevation axis relative to the horizontal plane and .theta..sub.S represents the satellite altitude angle.